Investigatorscanrecordanactionpotentialwithamicroelectrodewithoutpenetratingtheaxonbyplacingtwospacedelectrodesonitssurfaceandcomparingtheelectricalchargeateachpoint.Anelectrodelocatedwherethereisanactionpotentialwouldyieldasomewhatnegativesignalincomparisontoanelectrodewherethereisnoactionpotential.Astheactionpotentialisconductedtothesecondelectrode,thepolarityoftherecordingreverses.Thistechniqueisusedclinicallytoassessnervefunction.Peripheralnervesandmanycentralpathwaysconsistofapopulationofaxons ofvariousdiameters(Fig.5.12 );someoftheaxonsaremyelinated,andsomearenot.Consequently,actionpotentialstravelatdifferentvelocitiesintheindividualaxons.Asaresult,arecordingfromsuchanervewithexternalelectrodesdoesnotshowasinglesynchronouspeakbutaseriesofpeaksthatvaryintime(whichreflectstheconductionvelocityofgroupsofaxons)andinmagnitude(whichreflectsthenumberofaxonsineachvelocitygroup).Thisiscalledacompound action potential.Theclinicalvalueofsucharecordingisitsability,incertaindiseasestates,torevealthedysfunctionofaparticulargroupofaxonsassociatedwithspecificfunctions,aswellasthenoninvasivenatureofthetechniquebecauseitcanbeperformedwithskinsurfaceelectrodes( receptor or generator potentials. The receptor potentials can then be transformed into patterns of action potentials that are conducted over one or more axons into the CNS. In order for this to happen, the stimulus must produce receptor potentials that are large enough to change the spiking levels of one or more primary afferent fibers that are connected to the receptor. Weaker intensities of stimulation can produce subthreshold receptor potentials, but such stimuli do not change the activity of central sensory neurons and thus are not detected. Thus stimulus threshold is defined as the weakest stimulus that can be reliably detected.
Environmental events that evoke sensory transduction can be mechanical, thermal, chemical, or other forms of energy. However, the types of information used by a
CHAPTER 5 Generation and Conduction of Action Potentials particular organism depend on its set of sensory receptors.
For example, humans cannot sense electrical or magnetic fields, but other animals can sense such stimuli. In particular, many fish have electroreceptors, and various fish and birds use the earth’s magnetic field to orient themselves during migration.
The transduction process varies with the type of environmental stimulus being detected.
Fig. 5.13 shows three examples of how stimuli can alter the membrane properties of the specific sensory receptors that transduce such stimuli 0246 (further details for each of these examples are given in other 36 72 108 m/s chapters).
Fig. 5.13A illustrates how a chemoreceptor, such A as that used for taste and smell, might respond when a chemical stimulant reacts with receptor molecules on the plasma membrane of the sensory receptor. Binding of the •Fig. 5.12 Thedistributionofaxons,bysizeandconductionvelocity,inamixed(muscle)nerve(A) andacutaneousnerve(B). Notetheincreasednumberofsmall-diameterfibersandtheabsenceofAα fibersinthecutaneousnerve.(FromHainesDE[ed].Fundamental Neuroscience for Basic and Clinical Applications. 3rded.Philadelphia:ChurchillLivingstone;2006.) chemical stimulant to the receptor molecule opens an ion channel, which enables the influx of an ionic current that depolarizes the sensory receptor cell. (This is similar to what is described for ligand-gated channels in
Chapter 6.)
Fig. 5.13B, the ion channel of a mechanoreceptor, such as those in the skin, opens in response to the application of a mechanical force along the membrane, and this allows an influx of current to depolarize the sensory receptor. In